Aluminum is produced in Hall-Heroult cells by the electrolysis of alumina in molten cryolite, using conductive carbon electrodes. During the reaction the carbon anode is consumed at the rate of approximately 450 kg/mT of aluminum produced under the overall reaction ##EQU1##
The problems caused by the consumption of the anode carbon are related to the cost of the anode consumed in the reaction above and to the impurities introduced to the melt from the carbon source. The petroleum cokes used in the anodes generally have significant quantities of impurities, principally sulfur, silicon, vanadium, titanium, iron and nickel. Sulfur is oxidized to its oxides, causing particularly troublesome workplace and environmental pollution. The metals, particularly vanadium, are undesirable as contaminants in the aluminum metal produced. Removal of excess quantities of the impurities requires extra and costly steps when high purity aluminum is to be produced.
If no carbon is consumed in the reduction the overall reaction would be 2Al.sub.2 O.sub.3 .fwdarw.4Al+3O.sub.2 and the oxygen produced could theoretically be recovered, but more importantly with no carbon consumed at the anode and no contamination of the atmosphere or the product would occur from the impurities present in the coke.
Attempts have been made in the past to use non-consumable anodes with little apparent success. Metals either melt at the temperature of operation, or are attacked by oxygen or by the cryolite bath. Ceramic compounds such as oxides, with perovskite and spinel crystal structures usually have too high electrical resistance or are attacked by the cryolite bath.
Previous efforts in the field have resulted in U.S. Pat. No. 3,718,550, Klein, Feb. 27, 1973, Cl. 204/67; U.S. Pat. No. 4,039,401, Yamada et al., Aug. 2, 1977, Cl. 204/67; U.S. Pat. No. 3,960,678, Alder, June 1, 1976, Cl. 204/67; U.S. Pat. No. 2,467,144, Mochel, Apr. 12, 1949, Cl. 106-55; U.S. Pat. No. 2,490,825, Mochel, Feb. 1, 1946, Cl. 106-55; U.S. Pat. No. 4,098,669, de Nora et al., July 4, 1978, Cl. 204/252; Belyaev+Studentsov, Legkie Metal 6, No. 3, 17-24 (1937), (C.A. 31 [1937], 8384); Belyaev, Legkie Metal 7, No. 1, 7-20 (1938) (C.A. 32 [1938], 6553).
Of the above references Klein discloses an anode of at least 80%, SnO.sub.2, with additions of Fe.sub.2 O.sub.3, ZnO, Cr.sub.2 O.sub.3, Sb.sub.2 O.sub.3, Bi.sub.2 O.sub.3, V.sub.2 O.sub.5, Ta.sub.2 O.sub.5, Nb.sub.2 O.sub.5 or WO.sub.3 ; Yamada discloses spinel structure oxides of the general formula XYY'O.sub.4, and perovskite structure oxides of the general formula RMO.sub.3, including the compounds CoCr.sub.2 O.sub.4, TiFe.sub.2 O.sub.4, NiCr.sub.2 O.sub.4, NiCo.sub.2 O.sub.4, LaCrO.sub.3, and LaNiO.sub.3 ; Alder discloses SnO.sub.2, Fe.sub.2 O.sub.3, Cr.sub.2 O.sub.3, Co.sub.2 O.sub.4, NiO, and ZnO; Mochel discloses SnO.sub.2 plus oxides of Ni, Co, Fe, Mn, Cu, Ag, Au, Zn, As, Sb, Ta, Bi & U; Belyaev discloses anodes of Fe.sub.2 O.sub.3, SnO.sub.2, Co.sub.2 O.sub.4, NiO, ZnO, CuO, Cr.sub.2 O.sub.3 and mixtures thereof as ferrites, de Nora discloses Y.sub.2 O.sub.3 with Y, Zr, Sn, Cr, Mo, Ta, W, Co, Ni, Pa, Ag, and oxides of Mn, Rh, Ir, & Ru.
The Mochel patents are of electrodes for melting glass, while the remainder are intended for high temperature electrolysis such as Hall aluminum reduction. Problems with the materials above are related to the cost of the raw materials, the fragility of the electrodes, the difficulty of making a sufficiently large electrode for commerical usage, and the low electrical conductivity of many of the materials above when compared to carbon anodes.
U.S. Pat. No. 4,146,438 Mar. 27, 1979, de Nora, Cl. 204/1.5 discloses electrodes of oxycompounds of metals, including Sn, Ti, Ta, Zr, V, Nb, Hf, Al, Si, Cr, Mo, W, Pb, Mn, Be, Fe, Co, Ni, Pt, Pa, Os, Ir, Rh, Te, Ru, Au, Ag, Cd, Cu, Sc, Ge, As, Sb, Bi and B, with an electroconductive agent and a surface electrocatalyst. Electroconductive agents include oxides of Zr, Sn, Ca, Mg, Sr, Ba, Zn, Cd, In, Tl, As, Sb, Bi, Sn, Cr, Mn, Ti; metals Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Pd & Ag; plus borides, silicides, carbides and sulfides of valve metals. Electrocatalysts include Ru, Rh, Pd, Ir, Pt, Fe, Co, Ni, Cu, Ag, MnO.sub.2, Co.sub.3 O.sub.4, Rh.sub.2 O.sub.3, IrO.sub.2, RuO.sub.2, Ag.sub.2 O, Ag.sub.2 O.sub.2, Ag.sub.2 O.sub.3, As.sub.2 O.sub.3, Bi.sub.2 O.sub.3, CoMnO.sub.4, NiMn.sub.2 O.sub.4, CoRh.sub.2 O.sub.4 & NiCo.sub.2 O.sub.4.
Despite all of the above, preparation of usable electrodes for use in Hall cells still has not been fully realized in commercial practice. The raw materials are often expensive and production of the electrodes in the necessary sizes has been extremely difficult, due to the many difficulties inherent in fabricating large pieces of uniform quality.
Of the various systems disclosed above at this time no instance is known of any plant scale commercial usage. The spinel and pervoskite crystal structures shown above have displayed in general poor resistance to the molten cryolite bath, disintegrating in a relatively short time. Electrodes consisting of metals coated with ceramics have also shown poor performance, in that almost inevitably, even the smallest crack leads to attack on the metal substrate by the cryolite, resulting in spalling of the coating, and consequent destruction of the anode.
The most promising developments to date appear to be those using stannic oxide, which has a rutile crystal structure, as the basic matrix. Various conductive and catalytic compounds are added to raise the level of electrical conductivity and to promote the desired reactions at the surface of the electrode.